The present invention is directed to ink compositions and to processes for the use thereof. More specifically, the present invention is directed to compositions suitable for use in ink jet printing processes. One embodiment of the present invention is directed to an ink composition which comprises (1) water; (2) a colorant; and (3) an additive selected from the group consisting of oxy acids, oxy acid salts, and mixtures thereof.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
"Designer Solvents," M. Freemantle, Chemical and Engineering News, Mar. 30, 1998, pp. 32-37, the disclosure of which is totally incorporated herein by reference, discloses ionic systems consisting of salts that are liquid at ambient temperatures which can act as solvents for a broad spectrum of chemical processes.
Copending application U.S. Ser. No. 09/106,527, entitled "Inks for Ink Jet Printing With Reduced Intercolor Bleed," with the named inventor William M. Schwarz, filed concurrently herewith, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises water, an anionic dye, and a monoquaternary cationic penetrant of the formula ##STR1## wherein R.sub.1 is either a benzyl group or an alkyl group having at least about 3 carbon atoms, R.sub.2, R.sub.3, and R.sub.4 each, independently of the others, are hydrogen atoms, methyl groups, or ethyl groups, wherein two or more R groups can be joined together to form a ring, X is an anion, and n is an integer representing the charge on the anion, wherein the ink exhibits rapid penetration when applied to plain paper. Also disclosed is a set of inks for generating multicolored images which comprises (a) a first ink as described above; and (b) a second ink comprising water and a pigment; wherein intercolor bleed between the first ink and the second ink is reduced. Further disclosed are ink jet printing processes with the ink and ink set described above.
Copending application U.S. Ser. No. 09/106,396, entitled "Ink Compositions Containing Ionic Liquid Solvents," with the named inventor William M. Schwarz, filed concurrently herewith, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises water, a colorant, and an ionic liquid material. In a preferred embodiment, the ink is substantially free of organic solvents. Also disclosed is a process which comprises incorporating the ink composition into an ink jet printing apparatus and causing droplets of the ink composition to be ejected in an imagewise pattern onto a substrate.
Copending application U.S. Ser. No. 09/106,621, entitled "Ink Compositions Substantially Free of Organic Liquids," with the named inventors Kurt B. Gundlach, Maura A. Sweeney, Luis A. Sanchez, Richard L. Colt, and Melvin D. Croucher, filed concurrently herewith, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises water, an acid dye, a monovalent salt, a polyquaternary amine compound, and an optional surfactant, said ink being substantially free of organic solvents. The ink is particularly suitable for applications such as ink jet printing and marking pens. The disclosed inks in some embodiments are substantially indelible. Also disclosed is a composition for removing the ink compositions from substrates to which they have been applied which comprises water and a dianionic surfactant, optionally further containing a salt, urea, and/or a viscosity building agent such as a gum.
While known compositions and processes are suitable for their intended purposes, a need remains for improved ink compositions. In addition, a need remains for improved ink compositions suitable for use in ink jet printing processes. Further, a need remains for ink compositions containing improved humectants. Additionally, a need remains for ink compositions containing humectants which are compatible with a wide range of solvents. There is also a need for ink compositions which exhibit good heat stability. In addition, there is a need for ink compositions which enable generation of images with high optical density. Further, there is a need for ink compositions with desirable drying characteristics. Additionally, there is a need for ink compositions with good latency characteristics. A need also remains for ink compositions which can contain relatively high dye concentrations. In addition, a need remains for ink compositions which exhibit desirable interactions between the humectant and the colorant. Further, a need remains for ink compositions wherein the humectant is complexed with a cationic dye colorant. Additionally, a need remains for ink compositions which enable desirable interactions between the ink and a basic-sized paper substrate. There is also a need for ink compositions which reduce intercolor bleed. In addition, there is a need for ink compositions which enable improved edge sharpness.